In medical practice, respiratory assistance devices, such as an artificial ventilator, are used. As the respiratory assistance device, various methods are adopted including such methods as a controlled ventilation (Controlled Ventilation) method that is used for patients unable to breathe spontaneously (patients under general anesthesia, cardiopulmonary resuscitation, or in a critical condition), an assisted ventilation (Assisted Ventilation) method that generates a positive pressure (positive pressure) in an air passage in accordance with the spontaneous breathing of a patient, a partial assisted ventilation (Assist/Control) method that combines the assisted ventilation and the controlled ventilation, and a high frequency oscillation ventilation (high frequency oscillation) method that realizes an extremely low amount of one-time ventilation of 1 to 2 ml/kg by vibrating gas supplied by the air passage at a frequency of 5 to 40 Hz.
Each of these respiratory assistance devices requires a pump unit that generates the positive pressure in the air passage and an exhalation valve that discharges exhaled air to the outside air while maintaining the positive pressure.
The pump unit uses a relatively large device as a power source, such as a blower that transports a gas by rotating a fan and a cylinder pump that transports a gas by causing a piston to be reciprocated. Therefore, in conventional respiratory assistance devices, a box-shaped casing accommodating the pump unit is used while being installed beside a user.
There are some exhalation valves with a structure that is simplified by using a diaphragm (for example, see Japanese Patent Application Laid-Open No. Hei. 05-245204). The diaphragm is arranged so as to close an outlet of a pipe line that guides the exhaled air, and opens the pipe line by deforming in the direction moving farther away from the outlet. On the opposite side to the pipe line in the diaphragm, a back chamber to and from which the pump unit feeds and discharges air is provided. More specifically, the diaphragm is arranged so as to separate a space inside the pipe line from a space inside the back chamber and functions using a difference in air pressures between those spaces.
More specifically, when the air pressure inside the pipe line is lower than the air pressure inside the back chamber, the diaphragm adheres to and seals the outlet of the pipe line. On the other hand, when the air pressure inside the pipe line is higher than the air pressure inside the back chamber, the diaphragm deforms to the side of the back chamber and opens the pipe line to release the exhaled air to the outside air. According to the exhalation valve, even when a patient coughs or sneezes, since the diaphragm instantaneously responds thereto and the exhaled air is released to the outside air, the air pressure inside the pipe line does not increase excessively, and consequently, it is possible to inhibit placing an additional burden on the patient.